Insulating *Porch* Roofs with Leftovers… or Radiant Barrier?
Weird question. I’m building a house in Zone 2A – months of extreme heat/sun. The house’s (hip) roof will be insulated with 2 layers of 3 1/2″ polyiso. The finish roof will be white, high SRI standing seam.
There are 2 gabled porch roofs built off of the main roof. Of course, I won’t be covering those with polyiso – instead, to make them plane out with the building roof, I installed 7″ H blocks (so it looks like there’s a little floor framed atop each porch roof).
Normally I wouldn’t insulate this porch roof since… well… there are no walls. However, I have access to ~infinite *free* fiberglass batt cutoffs from a local fabricator. So I’m wondering if it’s worth a shot to reduce the sensation of heat radiating from the porch ceiling.
A few things I could do before adding the plywood (then the underlayment, then the standing seam):
1) Throw a ~3″ thick layer of fiberglass batt in each little “joist bay” on the porch roofs. I’d just delaminate my cutoffs and throw it in there, maybe stomp it down a bit. So, the FG wouldn’t be installed to a high spec, just kinda stuffed in there. (However, there would be an air barrier on both sides of the FG… just a lot of dead air space too).
2) Install the FG to a high spec (normal for me)… i.e. take the time to fill each 7″ high joist bay with fiberglass, which seems like quite a waste, but this is stuff that goes to the dumpster unless I take it from the guy. The R-value would be way higher than anything even reasonably needed of course. Would only do it if Option 1’s air pockets make it pointless.
3) Install a foil radiant barrier atop the blocks. Don’t worry, I don’t believe the radiant barrier hype. Just spitballing here since after this awful summer (105 F day after day), anything I can do to make the porch more comfortable is a good thing…
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
I would definately insulate the roof. I'm in much colder climate and have an uninsulated porch roof and you can feel the heat off it when the sun hits it.
It pretty much doesn't matter what you do up there as anything is better than the roughly R3 of the uninsulated porch roof.
Radiant barrier adds somewhere between R2 to R3 if you have an air gap, filling that air gap with fluffy adds about the same R value, so I would only do radiant barrier if it is pretty much free and zero extra work.
"... filling that air gap with fluffy adds about the same R value..."
Not sure what you mean here... the air gap I have is nearly 7" H. If I truly *filled* it, I'm sure it'd be higher than R-3 (has air-barrier on all 6 sides). Are you referring to my "option 1" above, where I just squish "some" FG in there (maybe a few inches)?
I would do at least 1. You can combine it with 3 but only if mostly free (ie OSB with built in radiant barrier. Option 2 doesn't sound like all that much extra work either but in this case you won't have a gap, so skip any radiant barriers.
jamesboris,
GBA regularly sees questions about how to convert porches into three-season rooms, and three-season rooms into fully conditioned living space. Would it make sense to do the exra work necessary (vent, air-seal the ceiling) to future-proof it?
Hi Malcolm! In this case, I'll say "probably not." These roofs cover a wraparound deck on an octagonal building. They cover large glass doors at the front & back of the building. And the wraparound deck is integral to the overall site design... it's hard to imagine walling it in. That's why I'm more focused on seeing how to make them more comfortable, wondering if you see a difference in my 3 options up there...
jamesboris,
Given that, I'd be tempted by option #1.